Quick change for super sponge

Just a little heat is enough to turn a waterproof block of foam into a sponge, a group of scientists reports.

Neil Shirtcliffe and his colleagues from Nottingham Trent University, UK, don't quite know what their nifty materials could be used for. The waterproof-sponge hybrids have been made to perform their trick at 400 °C or less, so the most likely application might be as a temperature sensor in an oven, they suggest. Regardless of use, the materials are an interesting chemical oddity.

The material is made from a matrix of methyltriethoxysilane filled with holes: more than 75% of the block's volume is air. The inside and outside surfaces are covered in a rough layer of water-hating methyl groups formed from carbon and hydrogen. This makes it so hydrophobic that water beads into little droplets on top of the foam, just as mercury does on a table.

But heating the foam beyond a critical temperature removes the methyl groups, leaving behind hydroxyl groups made from oxygen and hydrogen, which love water. This irreversibly turns the material into a sponge that sucks up drops of liquid.

Instant switch

The remarkable part is that the transition goes straight from waterproof to sponge with no middle step, says Carole Perry, who is part of the team.

If a sample is heated to 390 °C and then cooled, a drop of water will sit on top of the block. But a sample that has reached 400 °C before cooling will fully absorb the water. "We hadn't predicted how strong the effect would be before we saw it," says Perry. The group reports its results in Chemical Communications1.

The researchers have already made several materials with different switching temperatures, the lowest being 275 °C. Perry says that these materials could be used to record the maximum temperature reached in an oven: a strip of materials that switch at a range of temperatures could be soaked in coloured water after being taken out of the oven, she explains.

Those that had been activated would suck up the liquid, while those whose switching temperature was higher than the oven's maximum would remain dry. It is not clear, however, when such a thermometer would be better than the traditional sorts already used in ovens.

The group is now trying to find out whether the same chemistry could be applied to make a surface switch between being slippery and non-slippery.